One of the most important methods of preparing silicone polymers and copolymers is via ring-opening polymerization. Most commonly it is carried out by the polymerization of unstrained cyclosiloxanes, generally cyclic tetramers or pentamers, where no heat is generated upon polymerization. Typically the polymer constitutes up to approximately 90 percent of the resulting equilibrium mixture depending upon the substituents on the cyclosiloxane, the degree of polymerization achieved, and the amount of any solvent used in the polymerization. The amount of cyclosiloxanes that have been observed in equilibrium with many silicone polymers is described in Siloxane Polymers, Chapter 3, S. J. Clarson and J. A. Semlyen, ed., Ellis Horwood--PTR Rentis Hall, Englewood Cliffs, N.J., 1993. Polymerization can also be carried out with cyclic trimers. Cyclic trimers are strained and the polymerization is exothermic. Cyclic trimers are generally more difficult to prepare than cyclic tetramers and pentamers.
The most common fluorinated silicone is prepared by the ring-opening polymerization of 1,3,5-tris-(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane. The weight percent of the fluorocarbon portion of the cyclosiloxane and its resulting homopolymer is 44% by weight. The exothermic polymerization is driven due to the ring strain that is released upon the opening of the cyclic trimer. The polymerization must be stopped immediately after high polymer has formed, otherwise the polymer reverts to a mixture of cyclics, primarily the unstrained tetramer and pentamer which together constitute about 90% by weight of the cyclic mixture.
U.S. Pat. No. 5,202,453 describes a strained cyclotrisiloxane with a single fluorinated organic substitutent. This compound was prepared in a two step process from a 1H,1H,2H-vinyl terminated oligomer of hexafluoropropene oxide with a combined yield of approximately 50% based upon the moles of oligomer used. Though not demonstrated in the patent, it was suggested to be ring-opening polymerizable in the presence of alkaline or acid catalysts, presumably under conditions similar to those used for the polymerization of 1,3,5-tris-(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane.
Unstrained cyclosiloxanes with one or two large substituents on each Si atom typically are difficult to polymerize as the magnitude of a cyclosiloxane's equilibrium constant increases relative to that of a cyclosiloxane with smaller substituents, while at the same time the maximum concentration of the Si--O bond decreases as the size of the substituent increases. These factors result in a very high proportion of cyclosiloxanes at equilibrium as in the case of 1,3,5-tris-(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane. Because of this problem, it is common to prepare a silicone polymer with a small reactive group, often a hydrogen substituent, on the backbone. The hydrosilation of an olefin with a Si--H containing compound is frequently used to prepare organically substituted siloxanes. This hydrosilation is often difficult to perform if the solubility of the group to be substituted on the siloxane backbone is a low in the Si--H containing silicone polymer. Often a Si--H containing polymer contains sites where the Si--H has undergone hydrolysis to Si--OH during its preparation, storage, or during the hydrosilation reaction if water is present. Such units can permit the formation of branches or gels during the hydrosilation reaction. Perfluorinated organic molecules often display very low miscibility in silicone polymers. Hydrosilation reactions often do not proceed to complete conversion of the olefin or the silane reactants. For these reasons it is difficult to achieve a perfluorinated organic substituted silicone polymer by modification of a functional silicone polymer without defects in the structure due to incomplete reaction or defects from the starting Si--H containing polymer.
U.S. Pat. No. 5,247,116 describes a method where unstrained cyclosiloxanes with a single functional group can be prepared and subsequently isolated. This functional group may be transformed in a variety of fashions such that a single perfluorinated organic moiety may be introduced to a cyclosiloxane ring. The advantage of such a cyclosiloxane is that the Si--O bond concentration can be significantly higher than that of cyclosiloxane with a perfluorinated organic substituent at every Si atom, for an equivalent molecular weight of the cyclosiloxane. Furthermore, the equilibrium cyclization constants for the majority of the resulting cyclosiloxanes remain close to that of cyclodimethylsiloxanes. This results in a much greater fraction of linear siloxanes at equilibrium upon polymerization of the cyclosiloxane even though the starting cyclosiloxane is unstrained.